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may make it difficult for captive strains to be reestablished in the wild (S). Protection and restoration of natural habitats is the best and cheapest method of preserving the biological diversity and stability of the global ecosystem (2). Most theories of extinction deal with statistical properties of large assemblages of species, ignoring details of the species' ecology and population structure (6) and, therefore, these theories cannot pre- dict the extinction of particular species. With accelerating distur- bance of natural ecosystems by habitat alteration and introduction of exotic species, it is important to develop predictive models of extinction that can be used in programs to preserve or to control particular species. Souk and Simberloff (7) advocate an approach to the design of nature reserves that is based on target or keystone species instead of species diversity itself. Furthermore, much of the legal basis for conservation in the United States (the Endangered Species Act of 1973 and the National Forest Management Act of 1976) is oriented toward particular species rather than habitat types. The demographic and genetic consequences of population subdi- vision have been subjects of increasing interest among conservation- ists, although inbreeding depression and the maintenance of genetic variability, traditional subjects of popularion genetics, have recently received by far the most attention (8). This has led to relarive neglect of basic demography (the description and prediction of population growth and age structure), and conservation plans for some species have been developed primarily on population genetic principles. In this article I argue that demography may usually be of more c ate umpormnce popu atnon genetics in determining the nununiim V1 rzi s o wt u aborts. rrst review a genencs o inbreeding depression and the maintenance of genetic variability within populations. I then consider four demographic factors of fundamental imporrarnce for the survival of small populations. Finally I describe two management plans based on population genetics in which demographic principles were neglected with apparently dirt consequences for the species involved. 5- Population Genetics brbreedine depression. Historically large, that sudderilv decline to a few individuals usually experience reduced %iabilin• and fecundiav, known as inbreeding depression. In many species, lines propagated by continued brother-sister mating or sc - rtt rzanon tend to come st c or rm iablc after sev genera. pt in rce ng rn small pop abons p uces urg=ed 0mozygosity of (partially) recessive deleterious mutants that arc kept rare by selection in large populations, and by chance such mutations may become fixed in a small popularion despite counter- acting selection (9, 10). Detailed genetic analysis of Drosophila populations indicates that roughly half the inbreeding depression is due to individually rare, bur collectively abundant, nearly recessive lethal and semi-lethal mutations at about 5000 loci; individuals in large ourbred populations typically arc heterozygous for one or a few recessive lethals (11). The remaining inbreeding depression in Drosophila is caused by numerous slightly detrimental mutations that are mildiv recessive (12). It is not generally realized that gradual inbreeding or reduction of population size creates relatively little permanent inbreeding depression since selection tends to purge the population of deleterious recessive alleles when they become homo- zygous (9, 10), although the slightly detrimental, more nearly additive mutarions may be difficult (or impossible) to eliminate (12). Many invertebrate and plant species normally reproduce by sib- mating or self-fertilization; these have reduced, but appreciable, inbreeding depression manifested in hecrosis or hybrid vigor upon crossing different inbred lines (10, 12). r}t6 Managers of captive populations only recently became aware of the importance of avoiding inbreeding depression in propagating small populations (13). Now attempts frequently are made to minimize inbreeding and maYimizc genetic variability within popu- lations by transporting individuals (or gametes) long distances for breeding purposes (14), sometimes without sufficient attention to social factors or population structure and dispersal ability of the species in nature, or any attempt to gather or evaluate data on inbreeding depression (15). Some workers incorrectly assume that inbreeding depression is proportional to the mean inbreeding coetficient calculated from pedigree information or census data on a population (13, 16? and ignore the operation of selection during slow inbreeding. For species with an initial mean fitness high enough to withstand some inbreeding depression, even the fixation of a deleterious mutation should not preclude continued manage- ment of the population; for example, laboratory cultures of Drosoph- ila homozygous for major mutations not only can persist but often gradually reevolve the wild phenotype by natural selection of minor enetic modifiers (17). Genetic variation u4thin populations. In small populations, random fluctuation in gene frequencies (random genetic drift) tend, to reduce genetic variation, leading eventually to homozygosin, and the loss of evolutionary adaptability to environmental changes. The maintenance of genetic variability in a finite population can be understood through Wrighes concept of effective population size. This refers to an ideal population of N individuals with discrete generations reproducing by random union of gametes. The effective size of a population, N,, is the number of individuals in an ideal population that would give the same rate of random generic drift as in the actual population. Unequal numbers of males and females, increased variance in family size (greater than the mean), and temporal fluctuations in population size are the main factors causing the effective sizes of natural populations to be substantially less than their actual sizes (18). In the absence of factors acting to maintain genetic variation, such as mutation, immigration, or selection favoring hercrozygotes, the expected rate of loss of hetcrozygosity, or purely additive generic variance in quantitative characters, is 1/(21v,) per generation. Only a small fraction of the genetic variation will be lost on average in any one generation, because only rare alleles, which contribute little to hcrerozygosity or heritable variation in quantita- tive traits. are likely to be lost in a single generation of random sampling of gametes. However, small population size sustained for several generations can severely deplete genetic variability. Nonaddi- tive gene expression in quantitative characters within and between polymorphic loci (dominance and cpistasis) can cause transient increases in genetic variation in small populations (19), as can chance fluctuations in a purely additive genetic system, but this alone will not prevent the loss of most generic variability within about 2N, gencrarions. Using eidence that I compiled showing the high murability of quantitative characters in Drosophila, maize, and mice (20), Franklin (21) proposed that a population with an effccti<vc size of 500 could maintain typical amounts of heritable variation in sdectivel• neutral quantitative characters. This figure may be roughly correct even for characters under stabilizing natural selection favoring an intermedi- are optimum phenotype (3), but this does not justify its blanket application to species conservation. Since N, = 500 has been advo- cared as a general rule that gives the minimum population size for long-term viability from a genetic point of view (8, 21), it has been incorporated in species survival plans for both captive and wild populations (2-7-24), neglecting other factors, described below, that may require larger numbers for population persistence. Although quantitative (pol•genic) characters are of major impor- SCIENCE, VOL 24.1 _P x Burr hiss each even ?`?` gn a 16 si